There are six degrees of freedom to measure in an inertial guidance system. Acceleration in the x, y, and z directions, and rotation about the x, y, and z axis must all be measured. Sensors (accelerometers and gyros) are typically only sensitive in or about one axis. This means that the sensors must either be mounted to both vertical and horizontal surfaces within the guidance system (some tipped up 90 degrees) or a mix of sensors, some sensitive to inertia in one plane and some 90 degrees out of plane, must be used. Mounting sensors vertically is expensive, and mixing sensor types also mixes performance strengths and weaknesses in undesirable ways.
Another means of mounting sensors orthogonally to each other is with a “skew” mount. In this method, all sensors are tipped at an angle as though they were sitting on three faces of an imaginary cube, with the corner of the cube they all share pointing “straight up” or “down”. However, means for mounting sensors in a “skew” mount fashion are expensive, time consuming and require numerous manufacturing steps.